On April 16, 2001, at approximately 1800 mountain daylight time, a Mooney M20K, N921Z, was substantially damaged when the left main landing gear collapsed on landing at Skypark Airport, Bountiful, Utah. The commercial pilot and his three passengers were not injured. The airplane was being operated by the pilot under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the local flight which originated at 1735. No flight plan had been filed. According to the pilot's written statement, he departed runway 34 en route to Hailey, Idaho. Shortly after takeoff, he radioed Salt Lake City approach to request transition through class B airspace. Permission was granted and he was told to squawk 0306. He set the transponder to the given code and pressed the ident button. He was not "picked up" by approach so he repeated the process. After 2 to 3 minutes, the pilot said he "lost all electrical power." The pilot said, "I checked buses, recycled master and electrical system checked." He headed back to the airport and lowered the landing gear manually. He asked ground personnel to look to see if the landing gear was extended during a low pass, and they replied that it was. The pilot heard "pops in headset" so the master switch was kept off during landing. Upon landing, the left main landing gear collapsed, and the airplane skidded off the runway into a grass area. The right wing struck a sign post, which damaged the leading edge of the wing, and the fuselage was twisted near the empennage.